1. Field of the Invention
The present invention relates generally to mass spectrometry. The present invention relates more particularly to methods and systems for use in mass spectrometric identification of a variety of analytes, including high molecular weight species such as proteins and low molecular weight compounds like peptides, glycolipids and polyphenols.
2. Technical Background
In the field of proteomics and metabolomics, there exists a constant concern regarding the amount of sample available for analysis. Unlike genomics, in which samples may be amplified via polymerase chain reaction, in proteomics, the investigator is limited to the sample at hand. Accordingly, research has turned to the field of miniaturization technologies that enable the reduction of sample volume, thereby minimizing sampling loss in the handling of proteins and peptides. For example, minature fluid handling (microfluidic) systems have been built on planar substrates. Such so-called “lab-on-a-chip” systems have focused on small-scall mimics of traditional protein purification and separation methods, including the integration of affinity capture and capillary chromatography methodologies on the chip. The integration of functionalized microchannels and chemical reaction chambers that mimic protein/peptide fractionation by affinity capture or chromatographic separation to process peptides and proteins has become important in the desire to carry out single cell analysis.
Within the field of proteomics, mass spectrometry is a useful tool for protein identification and analysis. Accordingly, it is useful to interface lab-on-a-chip systems with mass spectrometers. Electrospray ionization (ESI) is a conventional method for transferring non-volatile compounds such as peptides and proteins to the gas phase for mass spectrometric detection. ESI is often used to couple real-time separation techiques (e.g., HPLC) with mass spectrometry. ESI can be advantaged in that it can produce precursor ions with higher order charge states (e.g., [M+nH]n+, where n>1) in order to provide more readily interpretable peptide tandem mass spectra, and thus allow peptide sequence to be assigned de novo or via a database search engine. However, ESI is disadvantaged in that it requires a capillary or nozzle for ionization. Such structures can be difficult to repeatably reproduce; accordingly, device-to-device variation can be significant. In turn, the conditions necessary to get a “Taylor cone” jet-and-plume structure desirable for ESI can vary significantly across devices. Moreover, ESI can be a relatively high-energy ionization process, and can therefore cause an undesired level of parent ion decomposition.
Matrix-assisted laser desorption ionization (MALDI) is another popular method transfer of peptides and proteins to the gas phase for mass spectrometry. Compared to ESI, MALDI is a “softer” ionization technique, generating primarily [M+H]+ ions. Moreover, where ESI generates ions continuously, MALDI is a pulsed technique that can allow separation to be decoupled from ionization. This decoupling can provide the opportunity to repeatedly re-examine a sample (e.g., to interrogate the evolution of a sample over time). MALDI, however, requires a matrix (often benzoic acid derivatives such as sinpainic acid), and that matrix provides contamination of the resulting mass spectrum at low m/z.
There remains a need for mass spectroscopy ionization techniques that address one or more of these deficiencies.